Python copy list issue

Question

I don't know what's wrong here, I'm sure someone here can help though. I have a list mylst (list of lists) that's being copied and passed into the method foo. foo iterates through the list and replaces the first element in the row with a passed in var and returns the altered list. I print the list and I see it gives me what I expect. I repeat that process again with another copy of mylst and a different passed in var. So the two returned lists should be different; however when I check the first list again I see that it's now the second list, also mylst has changed to that of the second list. Am I not copying the list correctly? I'm copying it with the mylst[:] method. Also another interesting observation is that all the list IDs are different. Doesn't that mean it's a different list than the others? Here's an example of my problem.

def printer(lst):
    print "--------------"
    for x in lst:
        print x
    print "--------------\n"

def foo(lst, string):

    for x in lst:
        x[0] = string

    print "in foo"
    for x in lst:
        print x
    print "in foo\n"

    return lst

mylst = [[1, 2, 3], [4, 5, 6]]
print "mylst", id(mylst), "\n"

first = foo(mylst[:], "first")
print "first", id(first)
printer(first) # Correct

second = foo(mylst[:], "second")
print "second", id(second)
printer(second) # Correct

print "first", id(first)
printer(first) # Wrong

print "mylst", id(mylst)
printer(mylst) # Wrong

Here's the print out on my computer

mylst 3076930092 

in foo
['first', 2, 3]
['first', 5, 6]
in foo

first 3076930060
--------------
['first', 2, 3]
['first', 5, 6]
--------------

in foo
['second', 2, 3]
['second', 5, 6]
in foo

second 3076929996
--------------
['second', 2, 3]
['second', 5, 6]
--------------

first 3076930060
--------------
['second', 2, 3]
['second', 5, 6]
--------------

mylst 3076930092
--------------
['second', 2, 3]
['second', 5, 6]
--------------

score 7 · Accepted Answer · answered Dec 02 '11 at 01:45

7

The lst[:] trick makes a copy of one level of list. You've got nested lists, so you may want to have a look at the services offered by the copy standard module.

In particular:

first = foo(copy.deepcopy(mylst), "first")

answered Dec 02 '11 at 01:45

Greg Hewgill

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do you know why the IDs are different? – Jeff Dec 02 '11 at 01:51
The IDs of the *first* list are different, but you'll find that the IDs of the lists *contained in* the first one are the same. – Greg Hewgill Dec 02 '11 at 02:21
`deepcopy` always makes the magic, I use it almost blindly in these scenarios – juliomalegria Dec 02 '11 at 03:36

score 0 · Answer 2 · answered Dec 02 '11 at 01:48

0

You aren't making another copy of mylist. Both times you call foo, you are passing the same object reference and modifying the same list.

answered Dec 02 '11 at 01:48

TJD

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Python copy list issue

2 Answers2

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